documentation: git-config minor cleanups

documentation: git-config minor cleanups

Change push.default's description to add hyphens between values and
descriptions to make the manpage easier to read.  The html version is
readable either way.

Change status.showUntrackedFiles to make item descriptions be
sentences and to use the same asciidoc format as push.default.  The
only visual change is the additions of "."

Signed-off-by: Cliff Frey <cliff@meraki.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>